Arianna Huffington signs off at The Huffington Post

Arianna Huffington, The Huffington Post's editor-in-chief, announced Thursday that she's leaving to head a new health, wellbeing and productivity startup.

"I thought HuffPost would be my last act," Huffington said in a tweet . "But I've decided to step down as HuffPost's editor-in-chief to run my new venture, Thrive Global."

The one-time conservative commentator oversaw explosive growth at the liberal online news and blog site that she co-founded in 2005, which went on to win a Pulitzer in less than a decade.

The site is known for its celebrity and newsmaker blogs and was a pioneer in the "aggregation" model in online news, posting stories that relied on articles and information from different news organizations as well as its own contributors. Initially, it was criticized for that practice by some journalists, including former New York Times editor Bill Keller . The site still has plenty of news from others, but it's also invested in original reporting. Within seven years of its founding, The Huffington Post had earned a Pulitzer Prize for national reporting on its series about wounded veterans. It was a 2016 Pulitzer Prize finalist for national reporting for a multimedia project on opioid addiction.

Aside from its U.S. edition, The Huffington Post has 14 international editions in multiple languages.

"People were mocking blogs and mocking celebrities and she built a formidable media and political force," said Jeff Jarvis, a journalism professor at CUNY in New York and well-known media commentator.

How or if The Huffington Post will change without its last remaining co-founder remains to be seen.

The company is part of much larger entity — phone and cable company Verizon, which is on an acquisition spree as it builds out its digital ad business. It bought AOL, Huffington Post's owner since 2011, last year. In late July, Verizon announced the $4.8 billion acquisition Yahoo, which has its own hefty media business.

Huffington said that the Yahoo buyout had nothing to do with her decision to leave, according to an article on The Huffington Post.

In a press release, AOL CEO Tim Armstrong said of HuffPost that AOL and Verizon "are committed to continuing its growth and the groundbreaking work Arianna pioneered."

For now, Huffington will be replaced by an interim editorial committee made up of five people, according to an internal memo posted on the site.

Jarvis doesn't believe HuffPost will change much since Huffington's main focus had shifted away from politics some time ago.

But Huffington has been very blunt in her distaste for Donald Trump, the Republican candidate for president. The Huffington Post, for almost five months in late 2015, covered Trump as entertainment news rather than politics. It moved coverage back to politics in December, announcing the change in a post penned by Huffington and titled "A Note on Trump: We Are No Longer Entertained."

Others are less sure the site will not change.

"There's a reason to fear that it wouldn't be as solid as it has been," said Jeff Cohen, an associate professor of journalism at Ithaca College, who started blogging on the site shortly after it was founded.

"I'm a little saddened," he said. "I think she's been a strong force for journalism."

Huffington's new venture, Thrive Global, will provide training, seminars and coaching about reducing stress and exhaustion. The company, which is expected to launch in November, shares a name with her well-being book "Thrive." Earlier this year she published "The Sleep Revolution," a book about getting more sleep.

Huffington said she couldn't run The Huffington Post and Thrive Global at the same time.

"Running both companies would have involved working around the clock," she said in a press release, "which would be a betrayal of the very principles of Thrive I've been writing and speaking about."

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This story is corrected to show that AOL bought The Huffington Post five years ago.